1. Field of the Invention
The invention relates to a method for producing polyesters, in particular polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and polytrimethylene terephthalate (PTT), and copolyesters as well as a device for carrying out the method. In particular, it involves a special embodiment of the device and method that relate to reducing pressure and producing vacuum in the reactors and pressure stages used.
2. Description of the Related Art
Multistage methods and devices for producing polyesters and copolyesters are known. A typical representative of such devices is composed, for example, of essentially two esterification reactors and two polycondensation reactors.
In a mixing device, an acid component and an alcohol component—in the case of PET, terephthalic acid and ethylene glycol—are mixed and then supplied to a first esterification reactor. In the production of PET, the reaction occurs there at approximately 240-280° C. and an elevated pressure and is then essentially completed in a second esterification reactor that operates at normal pressure.
The esterification reactors are usually connected to a rectification column, whose purpose is to separate out the large quantities of accumulated vapors, i.e. primarily the water produced during the reaction, from entrained monomer components—particularly ethylene glycol in the production of PET—in order to be able to convey the latter back to the esterification reaction. The gaseous water removed from the top of the rectification column is condensed and part of it is conveyed back into the column in the form of a reflux.
The product of the esterification reaction is then subjected to one or more prepolycondensation steps for further oligomerization. In this way, it is possible to achieve a sufficient viscosity for the final polycondensation, for which a disk ring reactor is often used. In this case, the condensation reactions, i.e. the prepolycondensation, the polycondensation, and final condensation, take place at reduced pressure or in a vacuum, which must be produced by means of vacuum units.
European patent EP 0244546 B1 has disclosed a method for producing high molecular polyester in which the esterification takes place in a two-stage or multistage reactor arrangement and the polycondensation then takes place under vacuum in at least one vacuum reactor. In this instance, the water vapor recovered from the esterification vapors by means of rectification is used as a driving medium for one-stage or multistage vapor jet vacuum units and is also used in fluid form for the condensation of process vapors in the mixing condensers situated after the water vapor jet pumps.
The disadvantage of this method lies in the fact that a part of the diol contained in the vapors of the vacuum stages is sucked in by the water vapor jets and lost. In order to partially counteract this, the condensation of the vapors must take place at very low temperatures. This leads to an increased precipitation of oligomers that clog the piping and the heat exchanger.
According to EP 0685502 B1, the vacuum in the polycondensation reactor is produced by glycol gas jet pumps. The latter are followed by glycol mixing condensers; the operating conditions for these are adjusted so that the glycol is condensed as completely as possible out of the vapors while low boiling reaction byproducts including water remain in the gaseous phase as much as possible and are only precipitated in a final condensation stage. The glycol precipitated in the mixing condensers is used on the one hand, after vaporization, as a driving medium in the gas jet pumps and on the other hand, as a condensation medium in the mixing condensers. The condensate from the last stage can be supplied, either without further purification or after distillation recovery, to the polyester production process, thus preventing excessive reaction byproduct from collecting in the glycol circuit. This method has the disadvantage that energy is required in order to produce the required glycol vapor as the driving medium for the vapor jet pumps.
DE 101 27 147 likewise describes a method for producing polyesters and copolyesters; this document, however, concerns itself solely with improving the part of the method relating to the esterification. The exceptional feature lies in the fact that with a pressure that falls successively from reaction stage to reaction stage, the vapor flows of the individual esterification stages are combined in a separating column and rectified. Since the pressure of the last esterification stage is less than the pressure of the rectification column, the vapors issLuing from the last esterification stage must be compressed to the column pressure. This can be carried out by means of compressors, blowers, or fans, or through the use of unrectified vapors as a driving medium of a vapor jet pump.